The objective of this study is to employ the methods of somatic cell genetics to investigate problems in the area of developmental genetics. Techniques in somatic cell genetics now provide analytical methods analogous to those used in defining functions of regulator genes in microbial organisms. These new methods are mutant selection, cell fusion (complementation analysis), and gene mapping with the aid of the mouse x human hybrid. First, cell fusion of two different cell types allows one to perform a dominance test regarding the expression of differentiated end-products. Secondly, the re-expression of a differentiated end-product with the loss of chromosomes in a somatic cell hybrid allows one to map regulator elements controlling the expression of the differentiated end-product in question. As a principal aim, human regulator genes will be examined through complementation analysis and gene mapping. This study will focus on the control of creatine kinase, alpha-glucan phosphorylase, actomyosin, and other markers in myoblast hybrids. Efforts will be made to devise selection systems for differentiated mammalian cells through the use of immunochemical, nutritional, and serum factor requirement differences between cell types. Methods devised for the selection of differentiated cell types should be of great aid in understanding growth control in vivo and malignancy.